[cmake] fix cubeb(?)

Signed-off-by: crueter <crueter@eden-emu.dev>

src/audio_core/externals/cubeb/CMakeLists.txt

@@ -6,12 +6,14 @@ set(CPM_USE_LOCAL_PACKAGES ON)
 CPMAddPackage(
     URI "gh:mozilla/cubeb#fa02160712"
     FIND_PACKAGE_ARGUMENTS "CONFIG" # not sure this works outside of gentoo
+    DOWNLOAD_ONLY YES
 )
 
 if (cubeb_ADDED)
     set(BUILD_TESTS OFF)
     set(BUILD_TOOLS OFF)
 
+    add_subdirectory(${cubeb_SOURCE_DIR} ${cubeb_BINARY_DIR})
     if (NOT MSVC)
         if (TARGET speex)
             target_compile_options(speex PRIVATE -Wno-sign-compare)